Most modern aircraft are powered by gas turbine engines, also known as jet engines. There are several types of jet engines, but all jet engines have certain parts in common. For example, all jet engines have an inlet with which to bring in free stream air into the engine. The inlet sits upstream of the compressor and, while the inlet does no work on the flow, there are important design features associated with the inlet. The total pressure through the inlet changes because of several flow effects. The inlet pressure performance is often characterized by the inlet pressure recovery, which measures the amount of free stream flow conditions that are recovered. This pressure recovery depends on a wide variety of factors, including inlet shape, aircraft speed, air flow demand of the engine, and aircraft maneuvers.
Flow field disturbances generated by fluid flow over aerodynamic surfaces within the inlet can buffet and fatigue downstream components exposed to these disturbances and reduce overall engine performance. Disturbances can also be ingested within engine air intakes leading to poor performance and/or stalling of the aircraft engines. This problem is exacerbated when the engine inlets have serpentine flow paths or exotic aperture shapes. These inlets and outlets may cause excessive propulsion performance losses. These losses emanate from strong secondary flow gradients in the near wall boundary of the airflow, which produce coherent large-scale vortices. Stalling the aircraft engine creates a potentially hazardous condition.
In the past, such problems have been solved by redesign of the inlet duct or redesign of the fan or compressor blades by adding dampening or increasing blade strength to change the natural frequency. Any of these changes may involve increased cost and weight associated with the aircraft.
Another solution employs passive vortex generator vanes to mitigate the effects of flow field vortices. However, these vanes result in increased weight and reduced performance over the engine's entire operating envelope. Vortex generators are small wing like sections mounted on an aerodynamic surface exposed to the fluid flow and inclined at an angle to the fluid flow to shed the vortices. The principle of boundary layer control by vortex generation relies on induced mixing between the primary fluid flow and the secondary fluid flow. The mixing is promoted by vortices trailing longitudinally near the edge of the boundary layer. Fluid particles with high momentum in the stream direction are swept along helical paths toward the duct surface to mix with and, to some extent replace low momentum boundary layer flow. This is a continuous process that provides a source to counter the natural growth of the boundary layer creating adverse pressure gradients and low energy secondary flow accumulation.
Yet another solution may employ variable geometries to alleviate these difficulties. Variable geometries allow the circumference of the inlet to vary thus changing the cross-sectional flow area. These solutions have many inherent disadvantages which penalize aircraft performance. For instance, variable geometry configurations can make up a significant portion of the weight of an engine. Such inlets are typically made of large, heavy metal flaps which mechanically alter their geometry by diverting fluid flow with physical blockage, and thus have to endure the high pressures associated with fluid flow. In these inlets, the actuators used to adjust the flaps to appropriate positions in the fluid flow tend to be heavy, expensive and complex because of the forces presented by the fluid flow which the flaps must overcome. Further, the flaps typically constrict the flow by closing and overlapping each other, which allows air to escape between the flaps. Variable geometry inlets are also difficult to implement on exotic nozzle aperture shapes.
Another method injects secondary flow(s) of high pressure air into the primary flow. Although the injection of a secondary flow will support a fixed geometry configuration, this method also introduces inefficiencies to the engine's operation. Injection of air across the flow tends to use a large amount of high pressure air. Thus, injection can introduce inefficiency because the total momentum of the fluid flow is decreased by the decreased flow from the compressor section into the combustion section if compressed air is bled from the compressor section for injection. This inefficiency can result in a reduced range of operations for a given fuel supply and a fuel flow.
New technology is therefore needed which will allow greater freedom to improve fluid flow within an engine inlet. Further limitations and disadvantages of conventional control surfaces and related functionality will become apparent to one of ordinary skill in the art through comparison with the present invention described herein.